Coating process



y 1960 J. T. 1 LOOMER 2,937,955

COATING PROCESS Filed Dec. 24, 1957 United States Patent O COATING PROCESS Joseph T. Loomer, Old Lyme, Conn., assignor to Continental Can Company, Inc., New York, N.Y., a corporation of New York Filed Dec. 24, 1957, Ser. No. 705,044

4 Claims. (Cl. 117-38) The present invention relates to a method and apparatus for the coating of paper, paperboard, and the like. Paper or paperboard coated in accordance with the present invention exhibits substantially improved characteristics of smoothness, printability and uniformity in color and density of the coating and lends itself to the development of extremely high gloss upon subsequent treatment in accordance with known processes. The process and apparatus of the present invention makes it possible to apply a smooth coating at speeds which are not necessarily ideal for the application of coatings by prior methods. Although it is not limited thereto, the process and apparatus may be embodied as an integral part of papermaking machines whether they be high-speed Fourdrinier machines or relatively slow cylinder machines of the type used for manufacturing boxboard.

Methods and apparatus heretofore used for the coating of paper or paperboard generally, and for machine coating in particular, may be broadly classified either as roll coating or doctor coating. In either of these broad categories the broad objective is to apply to a surface of the paper or paperboard a mineral pigment plus an adhesive in a liquid carrier, usually water.

Roll coating consists essentially of the formation upon the surface of a roll of a film of coating compound which is transferred to a traveling web of paper or paperboard by a process analogous to printing. Roll coating devices offer the substantial advantage of operating with coating compounds of relatively high solids content. That is, the coating compounds customarily used in roll coating may have solids content of from about 50% to about 70%. For this reason roll coating processes and apparatus may be quite readily integrated into a papermaking machine since they require a minimum of space and a minimum of additional drying capacity. Also, because of the high solids content there is no substantial wetting of the traveling paper web. Thus a freshly formed paper web may be subjected to smoothing in the press section, the smoothing press, and calender rolls, after which a relatively high solids content coating compound may be applied without danger of producing cockles or redevelopment of felt and wire marks or the production of blows or ply separation which are likely to occur when too much moisture is applied.

A great deal of development work has been done in connection with roll coating in an effort to produce acceptably smooth and uniform surfaces at the speeds encountered in the manufacture of certain grades of paper. However, it is a characteristic of roll coating that a pattern is formed, even the smoothest of such coatings having low and igh spots. Such pattern may take the form of pits, wells or ridges and is sometimes referred to as an orange peel effect. The pattern may come from a split ing of the film between the roll and the paper or it may come from the etched or satin finish imparted to the surface of the applicator or one of the transfer rolls. The pattern can be minimized by extremely careful selection of a combination of factors such as roll surface character, solids content and composition of the coating compound and speed of operation, and can be further minimized by subsequent calendering or remoistening plus calendering or by super-calendering. Carefully standardized procedures are customarily adopted for the production of coated paper which is acceptable for specific requirements. Such operation is quite inflexible and any changes in any one of the interdependent factors will almost inevitably result in the formation of a pattern which may be so prominent as to render the product unsatisfactory.

Doctor coating consists essentially of the application to a web of paper of a, layer of coating compound which is leveled by some form ofdoctor device which serves to distribute the coating compound over the surface of the paper in a layer of desired thickness. In the coating of paper the coating, compounds used in doctor coating processes are normally considerably lower in consistency than the compounds used in roll coating. For example, the solids content of doctor coating compounds usually runs from about 30% to about 55%. While theoretically a doctor device might be spaced from the surface of the paper by some exact predetermined distance so that a layer of uniform predetermined thickness could be extruded upon the paper, as a practical matter such result is seldom achieved with mineral coating compositions. Instead the doctor device is usually arranged to exert a certain amount of pressure upon the surface of the paper and the coating composition is more or less forced or trowled into the interstices of the paper and the top surface of the applied coating assumes a level approximating that of the highest spots on the paper base. Accordingly, doctor coating is normally utilized for the application of relatively light weight coatings which generally have little hiding power. Doctor devices of various kinds have been developed in the past and have found varying degrees of commercial acceptance. These include blades with sharp or rounded edges or rods sometimes wound with wire or small rolls, frequently rotating in one direction or another with respect to the advancement of the paper. Also, doctor coating may be effected by the use of a so-called air knife as is well known in the art. In some cases two or more doctor coatings are successively applied in an attempt to achieve greater hiding power. However, none of the doctor coating processes are normally used for the application of coatings of substantial thickness. Among other things the coating compositions suitable for doctor coatings are usually high in moisture content and a heavy coating thereof imparts too much moisture to the web.

From the above discussion it will be apparent that neither roll coating or doctor coating affords a practicable method for the application of a heavy mineral coating to a low cost product, such as paperboard at the speed of the paper machine .on which such products are manufactured, with assurance of obtaining a final surface sufficiently smooth for high grade printing or varnishing. The present invention affords a process by which a paperboard base of relatively low color may be coated, on the paper machine if so desired, with a mineral coating composition in sufiicient thickness to raise the color to almost any desired value and with a smooth surface capable of taking the finest printing and capable of being varnished to produce an exceptionally high gloss. To this end the present invention provides a process and apparatus by which a ground coat of high solids content mineral coating composition is applied by roll coating with deliberate, or at least inevitable, formation of a well defined pattern. The ground coat is then dried to a point such that it is dry to the touch and, preferably, while it is still warm from such drying step, an over coating of relatively low solids content mineral coating compound is applied by doctor coating. The doctor device is so adjusted as to level the overcoating approximately upon the high spots of the ground coat whereby substantially to fill all of the low areas in the pattern of the ground coat. In this manner a heavy relatively rough ground coat may be applied and relied upon to cover and thus to upgrade the color of the base sheet. The over coating may be relied upon to supply an unusually smooth and level surface to meet the demands of printing and finishing. This entire process may be carried out on the paper machine as a continuous process integrated with the manufacture of the base sheet and it may be carried out at a wide range of operating speeds including those practical for the manufacture of such relatively low priced products as the paperboard customarily used in the manufacture of folding boxes and the like.

It is an object of the present invention to provide a method and apparatus for the coating of paper or paperboard as described above. Other and further objects will become apparent on consideration of the following detailed description of preferred but not necessarily the only forms of the invention taken in connection with the drawings accompanying and forming a part of this specification.

In the drawing:

Fig. l is a diagrammatic illustration of the method and apparatus of the present invention;

Fig.2 is an enlarged and greatly exaggerated crosssectional view of a sheet of paper to which has been applied a ground coat in accordance with the present invention; and

Fig. 3 is a view similar to Fig. 2 showing an over coat applied over the ground coat in accordance with the present invention.

Referring now to the drawings, Fig. 1 illustrates an embodiment of the present invention in a papermaking machine. Thus a web of paper, paperboard or the like is continuously moving from the left, as viewed in Fig. 1, from the dryer drums (not shown) of a paperrnaking machine. The relatively dry web 10 is conducted over a stack of calender rolls 14, commonly known as a breaker stack. From the breaker stack the web 10 is conducted around a guide roll 16 and upwardly through a roll coating device 17 provided with a coating roll 18 and a backing roll 20.

The coating roll 18 and backing roll 20 may be of any desired or suitable construction for the purpose of applying to one surfaceof the web 10 a'heavy ground coat having a well defined pattern. -An exaggerated illustration of such ground coat is contained in Fig. 2 wherein a ground coat 22 having high spots 24 and low spots 26 is shown upon the web 10.

From the roll coating device 17 the web is conducted through suitable drying stages wherein the ground coat 22 may be dried at least to a condition known in the art as dry to the touch. For example, one or more dryer elements diagrammatically illustrated at '28 may be positioned immediately above the coating roll 18. The web 10 may then be conducted over a guide roll 30 to assume a generally horizontal path and further drying elements 32 may be disposed adjacent such horizontal path if desirable or necessary. The dryer elements 28 and 32 may comprise radiant or convection heaters or combinations thereof, as desired. The web It is then conducted over a backing roll 34 in such manner that the surface having the ground coat 22 thereon is disposed outwardly. A doctor coating device indicated generally at 36 is associated with the backing roll 34 for the purpose of applying a second layer of coating material referred to herein as an over coat. The doctor device serves to force or trowel the over coat material into the low spots or areas of the ground coat 22 and to level the surface of such over coat approximately even with the highest spots orareas of such ground coat 22. An exaggerated illustration of the relationship of such an over coat38 with the ground coat 22 upon the web 10 is contained in Fig. 3.

'tion.

manufacture of folding boxes.

After the application of the over coat in the doctor coating device 36, the web 10 may be conducted past one or more dryers such as 40, which may be of the same or different construction as the dryers 28 and 32. The capacity of the dryer or dryers 40 is desirably such that the two layers of coating material 22 and 38 will be rendered sufiiciently dry t'o'be safely conducted over the surface of a guide roll 42. From the guide roll 42 the web 10 may be conducted over one or more finishing calender stacks 44 and 46. From the last finishing calender stack the web 10 may be delivered to a reel or sheeter, as desired.

The roll coating device, as indicated above, may be of any desired construction and the illustration herein is primarily diagrammatic. Thus there is shown a trough 48 having a supply 50 of coating composition therein. A pickup roll 52 is arranged to be partially immersed in the supply of coating composition and a roll 54 serves to carry a film of coating composition from the pickup roll to the coatingroll 18.

. The doctor coating device 36 is similarly diagrammatically illustrated. It consists essentially of a doctor, shown as a blade 56, which is pressed edgewise against a portion of the web 10 which is supported by the backing roll 34. A supply or pond 58 of coating material is maintained on the upstream side of the doctor blade 56 whereby, in known manner, the web It) is conducted through the supply of coating material 58 and is thereafter leveled by the edge of the doctor blade 56.

It will be apparent that in addition to the selection of desired specific types of roll and doctor coating devices, there may be wide variation in the specific types and arrangement of other apparatus used in conjunction with the coating devices. Thus, as indicated above the capacity and type of drying equipment will vary with regard to the weight of coating applied, the moisture content thereof, speed of operation and similar factors. In some instances, it may be desirable to use steam heated dryer rolls instead of or in addition to such radiant or convected heat dryers as may be used. For example, the guide rolls 30 and 42 may be steam heated in the apparatus shown diagrammatically in Fig. 1.

Also, as will be more apparent from certain of the following examples of specific processes, the apparatus illustrated in Fig. 1 may be set up independently of a papermaking machine. In such event the web 10 may be fed from a reel and preferably will be fed over or through preliminary heating devices arranged ahead of the roll coating device 17.

The ground coat applied by the roll coating device .such as the device 17 may comprise a high consistency mineral coating composition, the low moisture content of which enables rapid drying without danger of disrupting the web or fibre formation. The over coat may comprise a relatively low consistency mineral coating composition, the moisture content of which will be readily absorbed by the preliminarily dried ground coat, again without danger of disruption of web or fibre forma- Thus, the previously applied and at least partially dried ground coat serves as a sort of filter cake upon the surface of which the solids contained in the over coating composition may be readily distributed and levelled by the doctor coating device.

While, as indicated above, the present process affords advantages which are of value inconnection with the coating of all kinds of paper products, several specific examples will be given of the use of this process in the coating of paperboard of the type frequently used in the Thus, in all of the following examples, the base stock consists of a multi-ply paper- .board formed upon a cylinder machine from pulp incorporating at least a substantial amount of fibre derived from waste paper. Thus, the base sheet is relatively low in color as will be apparent from the brightness range set forth below. In those examples in which the coating was applied in the paper machine, the operation was carried out at a normal speed of operation, that is to say, the incorporation of the present process into a continuous process for the manufacture of a particular grade of board or paper does not require any change in speed of operation. When the present process is practiced off the paper machine, the speed of operation is not critical and may be chosen at will within a wide range.

The roll coating device 17 may be any of the familiar types preferably utilizing a gravure roll arranged for indirect or direct transfer of a patterned coating to the web. The gravure roll may have a relatively smooth, metallic surface lightly etched to impart a satin finish or it may be quite deeply etched in a mesh or other pattern for the purpose of carrying the desired quantity of coating material to the web. The pattern on the surface of the gravure roll may consist of an overall pattern of depressions which may be discrete or interconnected as desired.

In the example chosen for illustration in Fig. l, the roll 54 is a gravure roll pressed into engagement with the pickup roll 52 and provided with a doctor blade 55 which serves in well known manner to control the quantity of coating composition carried by the gravure roll 54. This controlled quantity of coating composition is transferred to the coating roll 18, which in this instance may be a rubber-jacketed roll of the general type used in offset rotogravure printing. The coating composition is thus deposited upon the rubber surface of the roll 18 and in turn is transferred to the web as the latter passes between the coating roll 18 and backing roll 20.

As indicated above, the roll coating device 17 may be of the direct rotogravure type in which event the roll 54 will be a transfer roll, jacketed with rubber if so desired, and the coating roll 18 will be a gravure roll.

Depending upon the particular pattern on the gravure roll, whether it be the direct or indirect type, the ground coat applied to the web 10 may initially consist of a plurality of closely spaced discrete dots or it may consist of a generally continuous film with depressions. In either event, there will be a certain amount of levelling off of the coating on the web as a result of the liquid flow which occurs prior to drying. When roll coating alone is practiced the conditions must be carefully and rigidly maintained which secure maximum benefits from such leveling. In contrast with this the present process relies upon the roll coating device 17 to apply a coating of desired weight and having a surface pattern which would be regarded wholly undesirable for roll coating alone.

The doctor coating device may include a backing roll 34 having a smooth, metallic surface. Alternatively, the backing roll 34 may be jacketed with rubber or other yieldable material in accordance with the recommendations of many prior workers in the art of coating. Similarly, the doctor blade 56 may be a relatively rigid blade" rigidly positioned in desired proximity to the backing roll 34 or yieldably urged toward the backing roll as desired. Also it may consist of a relatively flexible blade capable of local yielding as is preferred by many workers in the art. Furthermore, the doctor 56 may consist of a rod or roller instead of the blade as illustrated. The doctor, whether it be blade, rod, roller or any other equivalent, may be yieldably urged by springs, compressed air or the like, into contact with the surface of the ground coat. In any event, the function of the doctor coating device is to force or trowel the over coating composition into the low areas of the ground coat and to level the material thus applied generally within-the plane of the higher spots in the ground coat.

It will be understood that Fig. 3 is intended to show the condition of the composite coatings 22 and 38 im- 'mediately. after the web leaves the doctor coating device 36. When the over coating 38 dries there will be some shrinkage, the degree of which will depend on the particular coating compositions used, and depressions will appear. However, the solids remaining in the over' coating 38 after drying will in great measure fill the depressions 26 of the ground coat and the final pattern achieved by this invention will always be substantially smoother than that achieved by the roll coating step above.

For the purpose of illustrating the versatility of and advantage secured by the present invention, several ex amples will be set forth. In each example the base sheet is a multi-ply boxboard formed on a cylinder machine from stock including a substantial amount of fiber derived from waste paper. Thus the base sheet has a low color requiring the application of a heavy opaque coating to bring it to acceptable brightness. plc the varnished samples tested were varnished under carefully controlled duplicate conditions. The gloss readings were taken on the well known Photovolt tester.

Example 1 This example was carried out on the paper-making machine as described above. The base coating composition was a mineral coating compostion containing clay and titanium dioxide with protein and latexas adhesives and with other additive materials of usual type. The base coating was applied at 61.5% solids and the same composition was diluted with water to 53.3% solids for the second or over coating.

The base coating was applied by an offset rotogravure coater in which the roll 43 was a gravure roll and the roll 18 was a rubber-jacketed offset roll.

The over coating was applied by a doctor coating device as described above wherein the doctor blade 56 was a flexible steel blade 0.012" thick pressed against the surface of the base coated sheet as it progressed around roll 34.

For the purposes of this example a portion of the base sheet was coated only in the roll coater 17 after which it was calendered in the stacks 44 and 46. Another portion was coated successively in the roll coater 17 and the doctor coater 36 after which it was calendered in the stacks 44 and 46. Samples of both portions were tested for gloss both before and after varnishing with the following results:

It will be obvious from the above example that the varnished gloss achieved with the two-step coating process of the present invention is definitely superior to that achieved by the single step of roll coating under the same conditions.

Further in connection with Example 1, the coating composition was removed from the doctor coater 36 and water was substituted. The balance of the operation was unchanged. In this test the water, applied at the doctor coater 36 remoistened the base coat and after drying and calenderi-ng the unvarnished gloss was improved slightly to 47.5 and the varnished gloss was up only one point to 84. This demonstrates that it is the application of an overcoating of coating material which produces the re markable improvement shown in this example rather than such remoistening of the base coat as is incidental to the application of the second coat.

Example 2 The procedure set forth in Example 1 was followed in its entirety except that a wire-wound rod was used for the doctor 56 instead of the flexible steel blade and the coating composition was diluted with water to a solids content of 49.8% for the over coat. The wire-wound rod was a commercially available article designed for In each exam-- doctor coating and known as a No. 6 Mayer metering rod. The glosstests were as follows:

Example 3 In the following example the base coating was ap plied by a roll coating device in the papermaking machine as described in Example 1. However, the basecoated web was dried and was not given an over coating and was not calcndered in the papermaking machine. Instead the base-coated web was wound on a reel with the base coat in its rough state. The reel of base-coated board was then unwound and conducted through a drying tunnel to preheat the web and the over coating was applied to the preheated web by means vof an air knife center of conventional construction and entirely separate from the paperrnaking machine. After the over coating had been applied the web was dried in a drying tunnel and passed through a super-calendering stack. Since the super-calendaring operation was carried out with the web in a fully dried condition, the gloss exhibited after supercalendering was considerably lower than that exhibited by the base coated web. Nevertheless the varnished gloss exhibited by the base sheet with base and over coat is very high as compared with the varnished gloss of the base sheet with base coating only. The gloss tests were as follows:

While throughout the above description reference has been made to drying or partial drying of the coating compositions and the apparatus has been described as including suitable drying equipment, it will be understood that such terms have been used for convenience in description. It is to be noted that liquid coating compositions usefulin this invention may solidify or partially solidify as a result of evaporation of a liquid fraction or the combining of a liquid fraction with other constituents, or both. Also the liquid fraction is not necessarily an aqueous liquid. Accordingly in the claims the coating composition is sometimes defined broadly as a settable liquid composition in order to refer to aqueous and nonaqueous compositions. Also the Words se setting" and variations thereof are used in the broad sense to refer to the solidification or hardening of aqueous or nonaqueous compositions by evaporation or combining or both.

What is claimed is:

l. A process for coating 21 web of paper, paperboard and the like which comprises forming a layer of coating material comprising a settable liquid mixture of coating solids and an adhesive on the surface of an applicator roll, transferring at least a portion of said layer to said web by pressure exerted between said web and the surface of said applicator roll while moving said web and said surface of said applicator roll at substantially identical linear speeds and in the same direction to imprint upon said web a first layer of coating material having a pitted surface characteristic of roll-applied coatings, at least partially setting said first layer of coating material upon said web, applying a second layer of coating material comprising a settable liquid mixture of coating solids and an adhesive tosaid pitted surface of said partially set first layer of coating material, wiping from said surface substantially all of said second layer of coating material which is in excess of that required to fill the pits in the surface of said first layer of coating material, and setting the composite coating.

2. A processfor coating a web of paper, paperboard and the like which comprises forming a layer of coating material comprising an aqueous mixture of coating solids and an adhesive on the surface of an applicator roll, transferring at least a portion of said layer to said web by pressure exerted between said web and the surface of said applicator roll while moving said web and said surface of said applicator roll at substantially identical linear speeds and in the same direction to imprint upon said web a first layer of coating material having a pitted surface characteristic of roll-applied'coatings, at least partially setting said. first layer of coating material upon'said web, applying a'second layer of coating material comprising an'aqucous mixture of coating solids and an adhesive to said pitted surface of said partiallyset first layer of coating material, wiping from said'surface substantially all of said second layer of coating material which is in excess of that required to fill the pits in the surface of said first layer of 'coating material, and setting the composite coating.

3. A process for coating a web of paper, paperboard and the like which comprises forming a layer of coating material comprising an aqueous mixture of coating solids and an adhesive on the surface of an applicator roll, transferring at least a portion of said layer to said web by pressure exerted between said web and the surface of said applicator roll while moving said web and said surface of said applicator roll at substantially identical linear speeds and in the same direction to imprint upon said web a first layer of coating material having a pitted surface characteristic of roll-applied coatings, at least partially setting said first layer of coating material upon said web, applying a second layer of coating material to said pitted surface of said partially set first layer of coating material, pressing a doctor blade against the surface of said first layer of coating material to wipe from said surface substantially all of said second layer of coating material which is in excess of that required to fill the pits in the surface of said first layer of coating material, and setting the composite coating.

4. A process for coating a web of paper, paperboard and the like which comprises forming a layer of coating material comprising an aqueous mixture of coating solids and an adhesive on the surface of an applicator roll, transferring at least a portion of said layer to said web by pressure exerted between said web and the surface of said applicator roll while moving said Web and said surface of said applicator roll at substantially identical linear speeds and in the same direction to imprint upon said web a first layer of coating material having a pitted surface characteristic of roll-applied coatings, at least partially setting said first layer ofcoating material upon said web, applying a second layer of coating material comprising an aqueous mixture of coating solids and an adhesive to said pitted surface of said partially set first layer of coating material, pressing a doctoring element against the surface of said first layer of coating material to wipe from said surface substantially all of said second layer of coating material which is in excess of that required to fill the pits in the surface of said first layer of coating material, and setting the composite coating.

References Cited in the file of this patent UNITED STATES PATENTS 1,360,763 Kirkpatrick Nov. 30, 1920 1,582,416 Mason Apr. 27, 1926 1,903,325 Colbert et a1 Apr. 4, 1933 

1. A PROCESS FOR COATING A WEB OF PAPER, PAPERBOARD AND THE LIKE WHICH COMPRISES FORMING A LAYER OF COATING MATERIAL COMPRISING A SETTABLE LIQUID MIXTURE OF COATING SOLIDS AND AN ADHESIVE ON THE SURFACE OF AN APPLICATOR ROLL, TRANSFERRING AT LEAST A PORTION OF SAID LAYER TO SAID WEB BY PRESSURE EXERTED BETWEEN SAID WEB AND THE SURFACE OF SAID APPLICATOR ROLL WHILE MOVING SAID WEB AND SAID SURFACE OF SAID APPLICATOR ROLL AT SUBSTANTIALLY IDENTICAL LINEAR SPEEDS AND IN THE SAME DIRECTION TO IMPRINT UPON SAID WEB A FIRST LAYER OF COATING MATERIAL HAVING A PITTED SURFACE CHARACTERISTIC OF ROLL-APPLIED COATINGS, AT LEAST PARTIALLY SETTLING SAID FIRST LAYER OF COATING MATERIAL UPON SAID WEB, APPLYING A SECOND LAYER OF COATING MATERIAL COMPRISING A SETTABLE LIQUID MIXTURE OF COATING SOLIDS AND AN ADHESIVE TO SAID PITTED SURFACE OF SAID PARTIALLY SET FIRST LAYER OF COATING MATERIAL, WIPING FROM SAID SURFACE SUBSTANTIALLY ALL OF SAID SECOND LAYER OF COATING MATERIAL WHICH IS IN EXCESS OF THAT REQUIRED TO FILL THE PITS IN THE SURFACE OF SAID FIRST LAYER OF COATING MATERIAL, AND SETTING THE COMPOSITE COATING. 